U.S. Pat. No. 4,369,745 to Howard, which issued Jan. 25, 1983, concerns an interlock circuit for a motor vehicle that is powered by an internal combustion engine. The internal combustion engine is coupled to a magneto ignition system and includes circuitry for inhibiting starting of the engine under certain conditions. The interlock circuit is electrically connected to an ignition switch and three safety switches. One safety switch opens when the transmission to a traction drive is engaged, and a second safety switch opens when a power take-off from the engine is engaged. The disclosed and preferred use of the ignition interlock of the '745 patent is with a riding lawn mower having a third safety switch which opens whenever the operator gets off the lawn mower.
The switches prevent operation of the lawn mower solenoid starter in the event an unsafe condition is sensed. The engine is also disabled subsequent to starting of the engine if the seat becomes unoccupied and either the transmission or power take-off is engaged. If both the transmission and power take-off are disengaged, the operator can get off the seat and the engine will continue to run. The disclosure of the '745 patent to Howard is incorporated herein by reference.
FIGS. 1a and 1b depict prior art safety interlock systems commercially available from the assignee of the present invention. An SCR device coupled to an engine magneto coil short circuits the magneto coil under certain conditions. Once the engine is running, the SCR turns on to deactivate the engine if the seat switch is open and one or both the transmission and power take-off switches are also open. If the seat switch opens and both the transmission and the power take-off switch are closed, the engine continues to run since this switch configuration means both the transmission and power take-off are disengaged and in a safe operating condition.
The circuit depicted in FIG. 1a responds to negative pulses from the engine magneto. The circuit depicted in FIG. 1b responds to positive pulses from the engine magneto. Thus, two separate circuits, one for the FIG. 1a embodiment and the second for the FIG. 1b embodiment are required to accomplish the same safety control function for different ignition systems.
Marlin Electric of Milwaukee, Wis. produces a commercially available circuit for disabling an engine. The circuit includes a triac that is activated by a battery voltage which is coupled to a triac control electrode when an unsafe condition is sensed.